Add listhost command

I'm missing an okconfig command line operation to check if a host is defined. I'm trying to confine myself to okconfig and not go directly via pynag. Does this make sense? This is kind of wingin' it, any input appreciated.

I did literally no testing on this, it might not even parse, as I don't have a proper test setup for okconfig. I know, my bad. But the tests here do not seem to be self contained, it looks like they pollute the host system. Am I misreading it?